1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte.
2. Description of the Background Art
Non-aqueous electrolyte secondary batteries, so adapted that a non-aqueous electrolyte is used, and lithium ions are transferred between a positive electrode and a negative electrode to carry out charge and discharge, are currently utilized as secondary batteries having high energy densities.
Generally used as such non-aqueous electrolyte secondary batteries is one using a lithium transition metal composite oxide such as LiCoO2 as a positive electrode, using a lithium metal, a lithium alloy, or a carbon material capable of intercalating and deintercalating lithium as a negative electrode, and using an organic solvent such as ethylene carbonate or diethyl carbonate in which an electrolytic salt composed of a lithium salt such as LiBF4 or LiPF6 is dissolved as a non-aqueous electrolyte.
In recent years, researches on the use of the non-aqueous electrolyte secondary batteries as power sources for machine tools and assist bicycles have been actively conducted by making use of high weight energy densities and high volume energy densities that characterize the non-aqueous electrolyte secondary batteries.
As positive electrode active materials (positive electrode materials) in the non-aqueous electrolyte secondary batteries used for these applications, a lithium cobalt composite oxide, a lithium nickel composite oxide, and a lithium manganese composite oxide have been examined. Among them, the lithium manganese composite oxide being abundant as resources and being low-priced have been paid attention to, and researches have been actively conducted toward commercialization. It is known that out of lithium manganese composite oxides, a lithium manganese composite oxide having a spinel structure is superior in high-rate discharge (large-current discharge) characteristics.
In the non-aqueous electrolyte secondary battery using lithium manganese oxide having a spinel structure as a positive electrode active material, however, deterioration of storage characteristics at high temperatures is a great problem. This is said to be because manganese (Mn) is dissolved from a positive electrode at the time of high-temperature storage.
In a case where lithium manganese oxide having a spinel structure is used as a positive electrode active material, deterioration of discharge characteristics after high-temperature storage becomes significant, and a voltage rapidly drops as discharge is started, resulting in a reduced discharge capacity. The voltage drops until it reaches not more than a discharge cut-off voltage and consequently, discharge becomes impossible to carry out. This is considered to be because manganese is dissolved from a positive electrode and is deposited on a carbon negative electrode so that the charge-discharge characteristics of the negative electrode are deteriorated.
Therefore, it is proposed that metal elements such as manganese composing a positive electrode active material are added to a non-aqueous electrolyte, to restrain dissolution of the metal elements in the positive electrode active material into the non-aqueous electrolyte in a chemical equilibrium manner (see JP 4-188571 A).
Even if the dissolution of the metal elements such as manganese from the positive electrode can be restrained, however, it is impossible to prevent the metal elements from being deposited on the carbon negative electrode. As a result of this, the charge-discharge characteristics of the negative electrode are deteriorated, as described above.
Therefore, it is proposed that metals such as titanium (Ti) and platinum (Pt) are carried on a surface of a negative electrode active material, to restrain deterioration of the charge-discharge characteristics of the negative electrode (see JP 2000-12027 A).
Furthermore, it is proposed that metal elements or metal ions such as cobalt (Co) are contained in predetermined amounts in a non-aqueous electrolyte, to improve the charge-discharge characteristics of a negative electrode (see JP 2003-217657 A).
In the non-aqueous secondary battery disclosed in JP 2000-12027 A, however, a method of carrying the metals such as titanium (Ti) and platinum (Pt) on the surface of the negative electrode active material makes it necessary to carry 0.5 to 20% by weight of the metals. Materials uninvolved in charge and discharge are thus used in large amounts, resulting in not only a reduced discharge capacity but also a greatly reduced energy density per unit weight.
The process of carrying the metals on the surface of the negative electrode active material is required, which causes cost to rise. Such a problem similarly occurs in not only lithium manganese oxide having a spinel structure from which much manganese is dissolved at the time of high-temperature storage but also all positive electrode active materials containing manganese as a constituent element.
Furthermore, in the non-aqueous electrolyte secondary battery disclosed in JP 2003-217657 A, it is difficult to significantly restrain voltage drop at the time of high-rate discharge (large-current discharge).